The invention of which the present disclosure is offered for public dissemination in the event adequate patent protection is available relates to hydraulic reservoirs or storage tanks from which the hydraulic pump of a heavy-duty hydraulic-power equipment draws hydraulic fluid which is to be pumped at high pressure to operate the equipment.
Modern usage of hydraulic circuits require special fluids for continued high performance, because of high speed and high pressures. This means that normal automobile type oils will not do the job. Automatic transmission fluids or its equivalent is required, even though the term "oil" is commonly used for them. This hydraulic fluid is not only very expensive, but hard to get in quantity, in out-of-the-way places. Hence, avoiding the loss of hydraulic fluid can be very important.
One limiting factor in all hydraulic circuits is the ability of the pump to get the fluid through the suction line so as to be able to pump it under high pressure to the point of work. The temperature and the viscosity and length and size and friction of the line are the deciding factors.
In order to help keep the pump charged, there has for many years been a well-known preference to place the hydraulic tank above the pump, or even to raise the hydraulic tank to the highest elevation that can reasonably be used. This gravity-aided feed, however, has a great disadvantage in that this head is always on the pump, and can, especially during an overnight stand, drain the tank when the pump's shaft seal leaks from wear or fatigue. The loss of oil is not always readily apparent, as the machine may be parked in a sandy area; and before it becomes apparent, the pump may cavitate, and destroy itself because of starvation or lack of oil. Another disadvantage of overhead oil tanks is that it is extremely difficult to change the seals of the pump, or even the pump itself. It is not permissible to provide a shut-off valve because of the danger of leaving it closed. Such a large vessel would be required to recover the oil that drains out of such a reservoir that recovery is rarely practical. Normal reservoirs contain a minute's supply of oil, so when this is 60 to 100 or more gallons of oil, handling it during repair becomes a major problem. It is not easy to store and keep clean such a large vessel, even in a well-equipped repair shop.
Loss of the hydraulic fluid from a tank can be a very serious problem except in the rare instances that a new supply is at hand. With nearly all machines, the engine direct-drives the pumps, with no drive-disconnect, so that the operator cannot even run the engines to get the machine to a repair shop without damaging the pumps because of not having them full of oil all the time.
Because of these problems with a raised tank, an alternative for many years has been to locate the oil tank below the level of the pump, but provide pressure in the tank to give extra help in getting the fluid to the pump. The common way to provide the pressure has been by having the tank generate pressure within itself. This is accomplished by having a combination relief valve and check valve in the top of the tank so that level changes in the tank due to movement of the motor pistons causes the tank to trap the difference of the volume of air caused by this movement.
Unused energy or friction of moving parts causes the trapped air to build up pressure more quickly by heating the air. Such pressure systems have also been used with elevated tanks, but even that did nothing toward removing the danger of loss of the fluid by an overnight leak. However, even with tanks lower than pumps, this system is not safe.
The danger, then, is that because there is a pressure head at the pump, the same as with the overhead tank, loss of the fluid may result. Even the part of the pressure due to heating the air in the tank may long remain. Oil that is not moving, as when the machinery is shut down, loses its heat very slowly.
The present invention removes the disadvantages of both systems by generating the necessary pressure, to help move the oil to the pump, only when the system is operating; by quickly removing the pressure when it is shut down; and by having the tank below the pump so there is no gravity head at any time.
The invention also has the advantage, over pressure developed by trapping and heating the air in a tank, that the pressure is provided more quickly and minimizes the drawing of air into the fluid when the shaft seal of the pump is imperfect. There has long been recognition that with unpressured equipment, a failing shaft seal will cause air to be drawn in at the hydraulic pump. Air in the oil is quite objectionable, and there have been efforts through the years to avoid or minimize its being drawn in. So far as known, all such efforts in the past have failed to achieve the aim dependably, or have had objectionable side effects.